A contact-type image sensor is used as a device for scanning a document in a facsimile machine, a copying machine, an image scanner or the like. The contact-type image sensor is provided with a line-illuminating device for linearly illuminating a document surface along a main scanning field.
In the line-illuminating device, a bar-shaped light guide is accommodated in a casing, light from a light source (LED) is introduced into the bar-shaped light guide, and the light is allowed to reflect within the bar-shaped light guide and be emitted from a light-emitting surface along the longitudinal direction of the bar-shaped light guide toward the document surface, in which the light-emitting surface is exposed from the casing (Patent Documents 1 and 2). There have been known a type in which a light source is provided on one end of the casing, and a type in which light sources are provided on both ends of the casing.
[Patent Document 1] Japanese Patent Application Publication No. 8-163320
[Patent Document 2] Japanese Patent Application Publication No. 10-126581
Among the conventional line-illuminating devices, as for the type in which a light source is provided on one end, the light amount is insufficient, and the illumination intensity is non-uniform along the main scanning direction. Thus, a light-scattering pattern is provided. However, it is still difficult to completely solve the problem of non-uniformity.
In contrast, as for the type in which light sources are provided on both ends, a problem is caused by thermal expansion. Specifically, while the temperature of the contact-type image sensor rises to quite a high temperature at the time of driving, it is cooled to around a room temperature at the time of stopping. The material of the casing is typically polycarbonate, and the material of the light guide is acrylic. Therefore, relative expansion and shrinkage repetitively occur between the casing and the light guide due to the difference in the thermal expansion coefficient. This results in the light guide shrinkage compared to the casing, which causes a gap between the end surface of the light guide and the light source, and part of light generated by the light source unpreferably leaks.